1. Field of the Invention
The present invention relates to a suction casting method and a suction casting apparatus.
2. Description of the Related Art
Conventionally, there is known, for example, an automotive wheel suction casting apparatus by means of a suction casting method for sucking molten metal from the pouring gate of a die into a cavity by the application of negative pressure, which structure is shown in FIGS. 6 and 7.
The structure is as follows. A lower surface plate 102 is provided at the upper portion of a temperature holding furnace 101 and a furnace-side pouring gate 103 is provided at the surface plate 102. A die 105 provided with a die-side pouring gate 104 communicating with the furnace-side pouring gate 103, is placed on the lower surface plate 102. A chamber 106 and an upper chamber 107 surround the outer periphery of the die 105. The pressure of interiors of the chambers 106 and 107 is reduced by a suction pump 108 (i.e., negative pressure is applied to the suction pump 108). Pressure P.sub.1 is applied to the interior of the cavity 112 equally through a clearance D.sub.1 between a side die 109 and an upper die 110 and a clearance D.sub.2 between the upper die 110 and a release pin 111, to thereby reduce the pressure of the interior of the cavity 112. Molten metal 113 within the temperature holding furnace 101 is sucked and filled into the cavity 112 through the furnace-side pouring gate 103 and the die-side pouring gate 104.
An upper surface plate 114 is fixedly attached to the lower surface plate 102. The upper chamber 107 and the upper die 110 are ascended and descended by being driven through a movable surface plate 116 by a hydraulic cylinder 115 disposed at the upper surface plate 114.
Also, there is provided suction means 118 for applying higher negative pressure than that of the interior of the cavity 112 to the abutment surface 117 between the furnace-side pouring gate 103 and the die-side pouring gate 104 using the suction pump 108, to thereby inhibit inflow of air from the abutment surface 117 into the pouring gates. The technique as described above is disclosed by, for example, Japanese Patent Publication No. 3-7465.
Meanwhile, the phenomenon of misrun occurs to a low temperature portion of the die stated above. As for the die 105, for example, short-run occurs to the surface portion of the upper die 110 lower in temperature than the side die 109 compared with the surface portion of the side die 109. Then, a space A is generated between the surface portion of the upper die 110 and a product 119 as shown in FIG. 7. As a result, the space A serves as an insulating layer to thereby cause casting defect (shrinkage) B shown in FIG. 7.
Furthermore, as stated above, even if applying high negative pressure to the abutment surface 117, the overall interior of the cavity 112 has the same pressure. Due to this, the air within the space A thus generated cannot be drawn out and removed, thereby causing casting defect B stated above as well.